1. Technical Field
The present disclosure relates to calibration systems and, more particularly, to a high-precision calibration circuit for calibrating an adjustable capacitance of an integrated circuit having a time constant depending on this capacitance.
2. Description of the Related Art
In the field of integrated circuits, the need is felt for providing integrated circuits having a preset time constant depending on the product RC of a capacitance value C multiplied by a resistance value R.
For example, in the state of the art, it is known to provide integrated circuits of which the cut-off frequency is a linear (particularly, inversely proportional) function of the product RC of a resistance R multiplied by a capacitance C. The process for producing these integrated circuits has inherent variations such that the product RC actually obtained can differ noticeably from the desired product RC. In some cases, the variations can be as high as 40% or 50% in module, and this entails that there may also be a factor of 3 between the lowest and highest cut-off frequencies of the filter achievable by means of the manufacturing process.
This gives rise to a problem, for example with a low-pass filter, in that:                when the value of the effective cut-off frequency of the filter is higher than the desired value, the filter can let undesired noise pass therethrough, mainly when the latter is present about the desired cut-off frequency;        on the contrary, when the value of the effective cut-off frequency of the filter is lower than the desired value, the useful signal is dampened.        
To solve these problems, several solutions have been developed that provide carrying out, after the integration process, the so-called trimming of the wafers on which the circuits are made. However, these solutions suffer from high costs in terms of additional area occupied, in terms of time required for testing the circuits, and further in terms of poor reliability. Furthermore, in some cases, these solutions are not feasible, mainly because the productive technology applied does not always provide for the components necessary for implementing these solutions.
An alternative solution to that mentioned above has been published in “IEEE JOURNAL OF SOLID-STATE CIRCUITS”, vol. 38, No. 5, May 2003, in the article “A Highly Integrated Analog Front-End for 3G”, and provides a self-adjusting circuit that, as compared with the trimming solutions described above, has lower consumption and lower area cost.
The self-adjusting circuit of the above-cited article is schematically represented in FIG. 1, in which it is generally designated with 1. In the article, the self-adjusting circuit 1, or calibration circuit 1, is used for adjusting the time constant RC of a low-pass filter 2, and is such to supply a calibration signal to this filter via the bus REG_BUS. The low-pass filter 2 is such as to receive an input signal to be filtered I_sig to output a corresponding filtered signal F_sig.
The calibration circuit 1 includes a calibration loop essentially comprising a switched capacitor array C_AR, an analogue comparator CMP, a control and timing logic unit TG_CNT and an interruption element SW_Res controllable by a signal Res_C and suitable to reset the switched capacitor array C_AR by short-circuiting the latter to ground.
The array switched capacitors C_AR can be selectively switched via a signal supplied by the control and timing unit TG_CNT to a bus C_BUS. The switched capacitor array C_AR is shown in FIG. 2a in greater detail.
In FIG. 2a, the array C_AR comprises four capacitors 8C, 4C, 2C, 1C, or modular capacitive elements, which are binary weighted relative to a unit capacitance of value C. Those skilled in the art will appreciate that the precision of the calibration circuit can be increased or decreased by providing a higher or lower number of modular capacitive elements, respectively.
The symbols B3, B2, . . . , B0 indicate the bits of the control signal provided to the array C_AR via the C_BUS, from the most significant bit to the least significant bit, respectively. And Sw3, . . . , Sw0 have been designated the switches for capacitors 8C, . . . , 1C, controlled by the respective bits B3, . . . , B0. These switches enable selective connecting and disconnecting (i.e., enabling and disabling) the capacitors of the array C_AR to and from node 5, the voltage of the latter being designated with VRC in FIG. 1. It should be observed that the effective capacitance Ceff in the capacitor array C_AR depends, in a given instant, on which and how many modular capacitive elements 8C, 4C, 2C, 1C are connected to the node 5 via the switches Sw3, . . . , Sw0. It should be further observed that the capacitor array C_AR comprises a fixed capacitor Cfx, which is the lowest capacitance value required for obtaining the calibration in the case where the product RC of the filter 2 such as obtained by means of the process has a highest value relative to the tolerance range normally ensured by the process. As is known to those skilled in the art, the total capacitance value that can be set in parallel with the fixed capacitor Cfx via the modular capacitive elements 8C, 4C, 2C, 1C must be established, on the contrary, such as to be capable of ensuring the granularity required by the calibration.
The calibration cycle carried out by means of the circuit in FIG. 1 is intended to identify by trial and error a control code B3, B2, B1, B0 (produced by the timing and control logic unit TG_CNT and provided thereby to the array C_AR via the bus C_BUS) which is such as to identify a set of array enabled capacitors C_AR such as to meet a preset calibration condition. At the end of the calibration cycle, the control code B3,B2,B1,B0 obtained is sent via the bus REG_BUS to the filter 2 and represents the calibration signal. In the filter 2, this code is applied to the variable capacitor Cvar in order to obtain a preset RC=R*Cvar product in the filter 2.
Waveforms illustrating the operation of the circuit in FIG. 1 are reported in FIG. 2b. 
With reference to FIGS. 1, 2a, 2b, the timing and control logic unit TG_CNT is such as to start a calibration cycle in response to a calibration-request signal C_REG and according to a timing imposed by a clock signal CK. At the beginning of each step of the calibration cycle, the timing and control logic unit TG_CNT is such as to short-circuit the array C_AR to earth, by closing the interruption element SW_Res, in order to discharge this array C_AR.
After the array has been discharged C_AR, the interruption element SW_Res is opened for a half-period of the clock signal and a certain number of modular capacitive elements 8C, . . . , 1C are connected to the node 4, by means of a selective closure of the switches Sw3, . . . , Sw0 that is commanded by the control code B3, . . . , B0.
These modular capacitive elements 8C, . . . , 1C start charging, thus causing the voltage VRC of node 5 to increase according to a transient characterized by a time constant equal to RF*Ceff and an asymptote given by the voltage Vcc. As stated above, Ceff represents the effective capacitance value (including the fixed capacitance value Cfx) of the array C_AR which are connected to node 4 at a given instant.
In a clock half-period, when the voltage VRC of node 4 reaches a voltage higher than voltage VCMP=VCC*R2/(R1+R2), the analogue comparator CMP will change its output state (in the example, the signal OUT_CMP will go from logic level “0” to logic level “1”).
The clock signal CK is a clock having a known frequency and there exists only one product RF*Ceff being such as to cause the voltage VRC to increase to the value VCMP in a half-period of clock signal CK. For this reason, the VCMP is set at a voltage value equal to the voltage that node VRC would achieve in a clock half-period according to a transient governed by a time constant 1/RC equal to, except for a scale factor, the desired value at which the time constant of filter 2 has to be calibrated.
In the particular circuit described in the above-cited article, the control codes sent from the timing logic unit TG_CNT to the array C_AR are generated by a counter of this unit, which is decremented at each step of the calibration cycle. The counter has a number of resolution bits equal to the number of the modular capacitive elements 8C, 4C, 2C, 1C that can be controlled by means of the control signal.
With particular reference to FIG. 2b, it should be observed that upon operation, before starting a calibration cycle, the signal RES_C short-circuits the capacitors Cfx, 8C, 4C, 2C, 1C of the array C_AR to ground until when the calibration cycle is started and the switches Sw3, . . . , Sw0 on the capacitors are such as to turn them off when they are driven with 0 or turn them on when they are driven with 1.
The switches Sw3, . . . , Sw0 are driven via the bits B3, . . . , B0 of the control code provided on the bus C_BUS. This code B3, . . . , B0 corresponds to the value of the unit counter TG_CNT. The counter, during the various steps of the calibration cycle, is decremented from the initial value 15 (in the case of 4 bits) by one unit at each period of clock signal CK.
At each decrement of the counter there corresponds a decrease, within the array C_AR, of a capacitive unit.
The first attempt (or first step) of the calibration circuit is carried out with all the capacitors of the array C_AR being enabled. When the level reached by the voltage VRC in a half-period of the clock signal CK is lower than the value of the voltage VCMP, the counter will be decremented by one unit. Consequently, also the subsequent control code B3, . . . , B0 will decrease by one unit, and thus the total capacitor of the array C_AR will be decreased by one elementary capacitor. This occurs, however, only after the array C_AR has been previously short-circuited to ground by means of the interruption element SW_Res in the half-period subsequent to the attempt described above.
During the subsequent steps, the calibration cycle is carried out by unit decrements until when the voltage value VRC achieved in one half-period is higher than the voltage VCMP. In the latter case, the cycle ends and the value reached in the counter is stored, which value will be then used as the code to be applied to the filter 2 for calibration and sent to the latter via the bus REG_BUS.
The prior solution described above suffers from several drawbacks. One of these drawbacks is that the comparator CMP has an inherent comparison delay that is higher as the voltage difference at the input thereof is lower. This delay is, accordingly, a non constant delay and further depends also on temperature and process.
Whereby, in the calibration circuit described above, when the voltage VRC exceeds the voltage VCMP before the end of the clock half-period in which the VRC is increased, but the change of the output of signal OUT_CMP outputted from the comparator CMP, due to this delay, is carried out after this half-period has ended, the logic and timing unit TG_CNT is not capable of appreciating this change of signal OUT_CMP, therefore an error is inevitably introduced in the calibration cycle. It should be observed that this problem is as much important as the clock frequency desired to be used is high.